docs: def cull_portals(self, stat, threshold=0.5, comparator=ge):
          """Delete portals whose stat >= ``threshold`` (default 0.5).
  
          Optional argument ``comparator`` will replace >= as the test
          for whether to cull. You can use the name of a stored function.
  
          """
          comparator = self._lookup_comparator(comparator)
          dead = []
          for u in self.portal:
              for v in self.portal[u]:
                  if stat in self.portal[u][v] and comparator(
                          self.portal[u][v][stat], threshold
                  ):
                      dead.append((u, v))
          self.remove_edges_from(dead)
          return self